This invention relates to the gasification of coal and similar carbonaceous materials and is particularly concerned with producing methanol by integrating a catalytic gasification process carried out in the presence of a carbon-alkali metal catalyst with a methanol synthesis process.
Existing and proposed processes for the manufacture of synthetic gaseous fuels from coal or similar carbonaceous materials normally require the reaction of carbon with steam, alone or in combination with oxygen, at temperatures between about 1200.degree. F. and about 2500.degree. F. to produce a gas which may contain some methane but consists primarily of hydrogen and carbon monoxide. This gas can be used directly as a synthesis gas or a fuel gas with little added processing or can be reacted with additional steam to increase the hydrogen-to-carbon monoxide ratio and then fed to a catalytic methanation unit for reaction with carbon monoxide and hydrogen to produce methane. It has been shown that processes of this type can be improved by carrying out the initial gasification step in the presence of a catalyst containing an alkali metal constituent. The alkali metal constituent accelerates the steam-carbon gasification reaction and thus permits the generation of synthesis gas at somewhat lower temperatures than would otherwise be required. Processes of this type are costly because of the large quantities of heat that must be supplied to sustain the highly endothermic steam-carbon reaction. One method of supplying this heat is to inject oxygen directly into the gasifier and burn a portion of the carbon in the feed material being gasified. This method is highly expensive in that it requires existence of a plant to manufacture the oxygen. Other methods for supplying the heat have been suggested, but these, like that of injecting oxygen, are expensive.
It has been found that difficulties associated with processes of the type described above, can largely be avoided by carrying out the reaction of steam with carbon in the presence of a carbon-alkali metal catalyst and substantially equilibrium quantities of added hydrogen and carbon monoxide. Laboratory work and pilot plant tests have shown that catalysts produced by the reaction of carbon and alkali metal compounds such as potassium carbonate to form carbon-alkali metal compounds or complexes will, under the proper reaction conditions, equilibrate the gas phase reactions occurring during gasification to produce methane and at the same time supply substantial amounts of exothermic heat within the gasifier. This additional exothermic heat of reaction essentially balances the overall endothermicity of the reactions involving solid carbon and thus results in a substantially thermoneutral process in which the injection of large amounts of oxygen or the use of other expensive methods of supplying heat are eliminated.
The catalytic effect of carbon-alkali metal catalysts on the gas phase reactions, as distinguished from the solid-gas reactions or the reactions of carbon with steam, hydrogen or carbon dioxide, allows the following exothermic reactions to contribute substantially to the presence of methane in the effluent gas and drastically reduces the endothermicity of the overall reaction: EQU 2CO+2H.sub.2 .fwdarw.CO.sub.2 +CH.sub.4 (exothermic) (1) EQU CO+3H.sub.2 .fwdarw.H.sub.2 O+CH.sub.4 (exothermic) (2) EQU CO.sub.2 +4H.sub.2 .fwdarw.2H.sub.2 O+CH.sub.4 (exothermic) (3)
Under the proper operating conditions, these reactions can be made to take place within the gasification zone and supply large amounts of methane and additional exothermic heat which would otherwise have to be supplied by the injection of oxygen or other means. Laboratory and pilot plant tests have shown that constituents of the raw product gas thus produced are present in equilibrium concentrations at reaction conditions and consist primarily of hydrogen, carbon monoxide, carbon dioxide, methane and steam.
It has been proposed in U.S. Pat. No. 4,211,669 to utilize steam gasification in the presence of a carbon-alkali metal catalyst to produce a chemical synthesis gas by treating the raw product gas withdrawn from the gasifier for removal of steam and acid gases, principally carbon dioxide and hydrogen sulfide; cryogenically separating carbon monoxide and hydrogen in amounts equivalent to their equilibrium concentration in the raw product gas from the methane in the treated gas; withdrawing the carbon monoxide and hydrogen as chemical synthesis product gas; contacting the methane with steam in a steam reformer under conditions such that at least a portion of the methane reacts with steam to produce hydrogen and carbon monoxide; and passing the effluent from the reformer into the gasifier. The reformer effluent will normally contain carbon monoxide and hydrogen in amounts equivalent to the equilibrium quantities of those gases present in the raw product gas and will therefore supply the substantially equilibrium quantities of hydrogen and carbon monoxide required in the gasifier along with the carbon-alkali metal catalyst and steam to produce the thermoneutral reaction that results in the formation of essentially methane and carbon monoxide.
As evidenced by U.S. Pat. Nos. 4,094,650 and 4,118,204, respectively, it has also been proposed to utilize steam gasification of a carbonaceous feed material in the presence of a carbon-alkali metal catalyst to produce both a high Btu and an intermediate Btu product gas. These processes are somewhat similar to the one described in U.S. Pat. No. 4,211,669. In the process disclosed in U.S. Pat. No. 4,094,650, the methane from the cryogenic separation step is recovered as product and the carbon monoxide and hydrogen are recycled to the gasifier to provide the required equilibrium quantities of hydrogen and carbon monoxide. In the process described in U.S. Pat. No. 4,118,204, the cryogenic separation step is eliminated and a portion of the carbon monoxide, hydrogen and methane exiting the acid gas removal step is recovered as the intermediate Btu product gas and the remainder is passed through a steam reformer to convert the methane into carbon monoxide and hydrogen. The effluent from the reformer is then passed into the gasifier to supply the required amounts of carbon monoxide and hydrogen.
Although the above-described catalytic gasification processes result in the substantially thermoneutral reaction of steam with carbon to form a raw product gas containing equilibrium quantities of carbon monoxide, carbon dioxide, hydrogen, steam and methane by recycling carbon monoxide and hydrogen in quantities equivalent to their concentration in the raw product gas to the gasifier and are therefore significant improvements over previously proposed non-catalytic and catalytic processes, they have one major disadvantage. None of the processes can be operated in a manner to produce liquid hydrocarbons. Since there may be a great need in the future for storable synthetic liquids that can be used to fuel vehicles, it may be highly desirable to utilize the thermoneutral process for gasifying carbonaceous materials described above in a manner which would allow the production of liquids instead of gases.